The present invention relates to an auxiliary front wheel drive system for a vehicle and more particularly relates to an auxiliary front wheel drive system for a motor grader.
The prior art includes various types of vehicles having main or primary drive systems for driving first sets of wheels by means of engine-driven mechanical transmissions, and auxiliary or assist drive systems for driving second sets of wheels by means of engine-driven hydrostatic transmissions when operating conditions are such that additional traction is desired. Representatives of these prior art vehicles are those respectively disclosed in U.S. Pat. No. 3,458,005 issued to Malm et al on 29 July 1969, U.S. Pat. No. 3,584,698 issued to Larson et al on 15 June 1971 and U.S. Pat. No. 3,997,017 issued to Campbell et al on 14 Dec. 1976.
The auxiliary drive system disclosed in the above-identified Malm et al and Campbell et al patents are similar to each other in that they both include electrical controls and utilize a variable displacement, pressure-compensated pump for delivering working fluid to fixed displacement hydraulic front wheel drive motors. Because it is a characteristic of such pumps to maintain the pressure of the working fluid at a preselected pressure and the pressure necessary for rotating the wheels depends upon the resistance encountered by them, then the pressure selected for maintenance by the pump must be high enough to rotate the front wheels at the highest expected resistance. The result of such systems has been that the front wheels tend to overspeed anytime the resistance to their rotation decreases substantially below an average resistance such as occurs, for example, when the wheels enter a mud puddle, are raised off the ground or encounter an icy surface. This overspeeding is undesirable since it results in undue engine horsepower consumption and working fluid heating and sometimes results in material such as gravel or mud and the like being thrown onto the vehicle and on any other thing that may be in the vicinity of the front wheels.
The auxiliary drive system disclosed in the above-identified Larson et al system avoids the overspeeding problem inherent in the Malm et al and Campbell et al auxiliary drive systems by employing a fixed displacement pump driven by the main drive transmission input shaft and having its displacement matched to that of the auxiliary drive motors such that, when the transmission is in its first speed range, the speed of the main drive wheels is required to be a preselected percentage greater than the speed of the auxiliary drive wheels before the pump has capacity to develop any driving torque in the motors, and, when the transmission is in its second speed range, the pump will have capacity to develop driving torque in the motors when the main and auxiliary drive wheel speeds are equal. However, the auxiliary drive system of Larson et al lacks versatility since the point at which the pump will supply sufficient fluid for developing driving torque in the auxiliary drive motors is very much dependant upon the speed ratios of the main traction drive transmission. In fact, the Larson et al auxiliary drive system includes a two-speed final drive transmission which is shifted in response to the main drive transmission being shifted to compensate for speed ratio changes in the main drive transmission.